Airplane air is clean but there are just too many people aboard and for too long. Something like a quarter of all air travel passengers get a respiratory illnes

Airplane air is clean but there are just too many people aboard and for too long. Something like a quarter of all air travel passengers get a respiratory illness within a week. Some hypothesize that the incredibly low humidity of cabin air (in most aircraft, except the very newest) makes it easier to acquire infections. A face mask, in addition to its normal role, also solves this problem because it is very humid inside the mask.